Sea Ice Thermodynamics
During the winter, sea ice acts like a blanket that insulates the relatively warm ocean water from the colder air above. As such, the "top" of the sea ice (i.e. closer to the atmosphere) is colder than the "bottom" (i.e. closer to the water). As you goes deeper into the ice, the temperature rises the closer you get to the water.
This is why the ice temperature graph shows colder (blue) colours closer to the surface and warmer (red) colours closer to the bottom!